Insulating film formed using an organic silane and method of producing semiconductor device

ABSTRACT

A silicon oxide film is formed to cover an island non-monocrystalline silicon region by plasma CVD using an organic silane having ethoxy groups (e.g., TEOS) and oxygen as raw materials, while hydrogen chloride or a chlorine-containing hydrocarbon (e.g., trichloroethylene) of a fluorine-containing gas is added to the plasma CVD atmosphere, preferably in an amount of from 0.01 to 1 mol % of the atmosphere so as to reduce the alkali elements from the silicon oxide film formed and to improve the reliability of the film. Prior to forming the silicon oxide film, the silicon region may be treated in a plasma atmosphere containing oxygen and hydrogen chloride or a chlorine-containing hydrocarbon. The silicon oxide film is obtained at low temperatures and this has high reliability usable as a gate-insulating film in a semiconductor device.

This application is a divisional application of U.S. Ser. No. 08/734,127filed Oct. 21, 1996, now U.S. Pat. No. 5,866,932, which is acontinuation of 08/455,574 filed May 31, 1995, now abandoned, which is adivisional of Ser. No. 08/198,054 filed Feb. 18, 1994 now abandoned.

FIELD OF THE INVENTION

The present invention relates to a method for producing agate-insulating film which is used in a thin film device such as agate-insulated field effect transistor or the like, at a low temperatureof 650° C. or lower, and also to the insulating film produced by themethod.

BACKGROUND OF THE INVENTION

Heretofore, in a thin film device such as a gate-insulated field effectthin-film-transistor (TFT) or the like, a silicon oxide film with goodcharacteristics, which is obtained by forming a crystalline siliconfollowed by heating and oxidizing its surface at high temperaturesfalling within a range of from 900 to 1100° C., has been used as agate-insulating film.

The oxide film formed by such thermal oxidation is essentiallycharacterized in that its interfacial level density is extremely low andthat it may be formed on the surface of a crystalline silicon at auniform thickness. Accordingly, the former brings about good on/offcharacteristics and long-term reliability on bias/temperature; while thelatter reduces the short circuit between a gate electrode and asemiconductor area (active layer) at the edges in an islandsemiconductor region to thereby improve the production yield ofsemiconductor devices.

To use such a thermal oxide film in producing semiconductor devices,however, a material which is resistant to high temperatures must beselected as the material for the substrate. In this respect, sinceinexpensive glass materials (for example, alkali-free glass such asCorning 7059, etc.) cannot be used, the production costs aredisadvantageously high especially when large-area substrates are used.Recently, a technical means for forming TFT on an alkali-free substrateis being developed, in which, however, a thermal oxide film cannot beused but a gate-insulating film shall be formed by sputtering or byphysical or chemical vapor deposition (CVD) such as plasma CVD orreduced pressure CVD.

However, it was inevitable that the characteristics of the silicon oxidefilm formed by such means were inferior to those of the thermal oxidefilm. Namely, the interfacial level density of the former is generallylarge and, additionally, the former was always accompanied by thedangers of alkali ions such as sodium ions or the like invading the filmbeing formed. In addition, since the step coverage of the silicon oxidefilm is not so good, the film frequently caused the short circuitbetween the gate electrode and the active layer at the edges of theisland semiconductor region. For these reasons, it was extremelydifficult to obtain semiconductor devices of the kind satisfying all thecharacteristics, the reliability and the production yield by the priorart technology.

SUMMARY OF THE INVENTION

The present invention has been made so as to solve at least one of theseproblems in the prior art technology. Accordingly, one object of thepresent invention is to provide a method for producing a silicon oxidefilm with good step coverage. Another object of the present invention isto provide a silicon oxide film which is resistant to unfavorableimpurities such as alkali ions and others and also to provide a methodfor producing the film.

First, the present invention is characterized in that a film which hasbeen obtained by plasma CVD using a mixed gas containing an organicsilane having ethoxy groups, oxygen, and hydrogen chloride or achlorine-containing hydrocarbon, as the raw material gas, and consistsessentially of silicon oxide is used as a gate-insulating film.

Secondly, the present invention is also characterized in that a filmwhich has been obtained by plasma CVD using a mixed gas containing anorganic silane having ethoxy groups, oxygen, and a fluorine-containinggas (e.g., NF₃, C₂ F₆), as the raw material gas, and consistsessentially of silicon oxide is used as a gate-insulating film.

Accordingly, the present invention provides an insulating filmconsisting essentially of silicon oxide, which has been formed on anisland non-monocrystalline semiconductor region consisting essentiallyof silicon to closely cover the region and is characterized in that from1×10¹⁷ to 5×10²⁰ cm⁻³ of halogens are detected from the film bysecondary mass spectrometry and that 5×10¹⁹ cm⁻³ or less carbons aredetected therefrom.

The present invention also provides a first method of producing asemiconductor device comprising a first step for forming an islandnon-monocrystalline semiconductor region consisting essentially ofsilicon and a second step for forming a film consisting essentially ofsilicon oxide over the non-monocrystalline semiconductor region in aplasma atmosphere resulting from a mixed gas containing an organicsilane having ethoxy groups, oxygen, and hydrogen chloride or achlorine-containing hydrocarbon.

The present invention further provides a second method of producing asemiconductor device comprising a first step for forming an islandnon-monocrystalline semiconductor region consisting essentially ofsilicon, a second step for exposing the island semiconductor region to aplasma atmosphere containing oxygen, and hydrogen chloride or achlorine-containing hydrocarbon, and a third step for forming a filmconsisting essentially of silicon oxide over the non-monocrystallinesemiconductor region in a plasma atmosphere resulting from a mixed gascontaining an organic silane having ethoxy groups and oxygen.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1(A) is a conceptual cross-sectional view showing a positive columnCVD apparatus used in an example of the present invention.

FIG. 1(B) is a conceptual plan view showing the positive column CVDapparatus shown in FIG. 1(A).

FIGS. 2(A) to 2(E) shows a flow sheet showing the formation of TFT inthe example.

FIG. 3 shows the characteristic curves of breakdown voltage of theinsulating films obtained in the example.

FIGS. 4(A) and 4(B) show the characteristic curves of V_(FB) of theinsulating films obtained in the example.

DETAILED DESCRIPTION OF THE INVENTION

As the organic silane having ethoxy groups, preferred are substances tobe represented by chemical formulae Si(OC₂ H₅)₄ (tetraethoxysilane,hereinafter referred to as TEOS), Si₂ O(OC₂ H₅)₆, Si₃ O₂ (OC₂ H₅)₈, Si₄O₃ (OC₂ H₅)₁₀ and Si₅ O₄ (OC₂ H₅)₁₂. Since these organic silanematerials move on the surface of the substrate for a long period of timeto be decomposed on the surface to form a silicon oxide film thereon,they may well get into even hollows to give an excellent film with goodstep coverage.

As the chlorine-containing hydrocarbon, preferred are substances to berepresented by chemical formulae C₂ HCl₃ (trichloroethylene), C₂ H₃ Cl₃(trichloroethane) and CH₂ Cl₂ (dichloromethane). The chlorine-containinggas of the kind is decomposed essentially in a vapor phase to becompounded with alkali elements such as sodium or the like that existsin the filming atmosphere, whereupon the resulting compound removes fromthe substrate to accelerate the removal of alkali elements from thesilicon oxide film being formed. Some chlorine atoms still remain in thesilicon oxide film formed, and these function as a barrier thereinagainst alkali elements which will try to invade the film from theoutward later on. As a result, the reliability of TFT may be improved.The concentration of the chlorine-containing hydrocarbon is preferablyfrom 0.01 to 1% relative to the whole mixed gas. If it is more than 1%,the gas will have bad influences on the characteristics of the filmformed.

In the insulating film consisting essentially of silicon oxide, thusformed by the above-mentioned method, halogen elements (e.g., fluorineor chlorine) are detected in an amount of from 1×10¹⁷ from 5×10²⁰ cm⁻³as impurity elements by secondary ion mass spectrometry, while thecarbon concentration is 5×10¹⁹ cm⁻³ or less. In particular, in order tolower the interfacial level density of the film, it is desired that thecarbon concentration is 1×10¹⁸ cm⁻³ or less. In order to lower thecarbon concentration, the temperature of the substrate during filmingmay be 200° C. or higher, preferably 300° C. or higher.

On the insulating film to be formed in this manner, many dangling bondsare often precipitated in the initial stage of its filming. Therefore,it is preferred to previously expose the substrate semiconductor layer(this preferably consists essentially of silicon) to a plasma atmospherecontaining oxygen. As a result, the interfacial level density is loweredwhile the fluctuations of the flat band potential in thebias/temperature test are reduced, and therefore the reliability of thesemiconductor device to be formed is improved. It is also preferred toadd to the atmosphere hydrogen chloride or a chlorine-containingmaterial such as trichloroethylene, trichloroethane, dichloromethane orthe like, in addition to oxygen, to further improve the effect.

On the other hand, after the formation of the insulating film consistingessentially of silicon oxide by the above-mentioned method, the film mayfurther be heat-treated at temperatures falling within the range of from200 to 650° C. to thereby reduce the fluctuations of the flat bandpotential. The heat treatment is preferably conducted in an oxygen-freeatmosphere such as argon, nitrogen or the like. The fluctuations of theflat band potential are noticeably reduced by the heat treatment at 450°C. or higher and the reduction is saturated at 600° C. or higher.

The second method of the present invention is characterized bycomprising exposing an island non-monocrystalline semiconductor regionconsisting essentially of silicon to a plasma atmosphere containingoxygen, and hydrogen chloride or a chlorine-containing hydrocarbon,followed by forming a film consisting essentially of silicon oxide overthe non-monocrystalline semiconductor region by plasma CVD using a rawmaterial containing an organic silane having ethoxy groups and oxygen.

In the second method, hydrogen chloride or a chlorine-containinghydrocarbon is essentially accumulated in the chamber during the plasmatreatment, which brings about the same effect as that to be broughtabout by the above-mentioned first method where hydrogen chloride or achlorine-containing hydrocarbon is added, during the following step offorming the silicon oxide film. The same as that mentioned above shallapply to the second step with respect to the improvement in thereliability attainable by the plasma treatment. To obtain a betterresult from the second method, it is also preferred that the chlorineconcentration and the carbon concentration in the silicon oxide filmthus formed by the second method are the same as those in the filmformed by the above-mentioned the first method. It is also preferred inthe second method that the film consisting essentially of silicon oxidethus formed is subjected to heat treatment at 200 to 650° C., preferablyat 450 to 600° C., after the filming in order to obtain a further betterresult.

The plasma CVD apparatus to be employed in the present invention may beeither an ordinary parallel plate-type apparatus (in which a pair ofelectrode plates are located in a chamber, facing to each other, and oneor both of them has/have a sample substrate mounted thereon) or apositive column-type apparatus such as that used in the followingexample.

However, the latter is preferred to the former in view of the followingtwo points. One is that the amount of the substrates to be treated atone time is determined by the area of the electrodes used in the former,while it is determined by the discharging volume in the latter.Accordingly, a larger amount of substrates may be processed at one timeby the latter. The other is that the surface of the substrate treated bythe former is much damaged by the plasma, while the latter is almostfree from the damage by the plasma since it has almost no potentialinclination. In addition, since the uniformity of the film to be formedusing the latter is better than that using the former, the uniform filmhas no bad influences on the characteristics of TFT and the productionyield thereof.

It is necessary that the chamber of the plasma CVD apparatus to be usedfor the filming in the present invention is sufficiently cleaned, priorto its use, so as to reduce the content of alkali elements, such assodium, etc., in the chamber. To clean the chamber, chlorine, hydrogenchloride or the above-mentioned chlorine-containing hydrocarbon may beintroduced into the chamber along with oxygen, and thereafter the plasmamay be generated therein. It is preferred that the chamber is heated at150° C. or higher, preferably 300° C. or higher, so as to moreeffectively carry out the step.

EXAMPLE

This example demonstrates one embodiment of the present invention offorming a silicon oxide film, as the gate-insulating film, on an islandnon-monocrystalline silicon semiconductor film by positive column plasmaCVD, essentially showing the electric characteristics of the siliconoxide film formed. The plasma CVD apparatus used herein is shown inFIG. 1. FIG. 1(A) is a vertical cross-sectional view of the apparatus,and FIG. 1(B) is a top plan view of the same. The positive column CVD ischaracterized in that the substrate to be coated is located in thepositive column region for plasma discharging and is coated with filmstherein.

The RF power sources 102 and 103 give the power to generate plasma.Regarding the frequency from the sources, radio waves are typicallyemployed, having a frequency of 13.56 MHz. The power fed from the twopower sources is adjusted by the phase shifter 104 and the matchingboxes 105 and 106 in such a way that the condition of the plasma to beformed is the best. The power derived from the RF power sources arrivesat the pair of electrodes 107 and 108 that have been located in parallelto each other in the inside of the chamber 101 and have been protectedby the electrode covers 112 and 113, thus causing discharging betweenthese electrodes. Substrates to be treated are located between theelectrodes 107 and 108. In order to improve the mass-producibility, thesubstrates 111 are cased in a container 109, where they are attached tothe both surfaces of the sample holders 110. The substrates arecharacterized in that they are parallel to each other between theelectrodes. The substrates are heated by the infrared lamp 114 and keptat suitable temperatures. Though not shown, the apparatus is fitted witha gas exhauster and a gas-feeding means.

The filming conditions and the characteristics of the film formed arementioned below. The temperature of the substrates was 300° C. Into thechamber, 300 SCCM of oxygen, 15 SCCM of TEOS and 2 SCCM oftrichloroethylene (hereinafter referred to as TCE) were introduced intothe chamber. The RF power was 75 W, and the whole pressure was 5 Pa.After the filming, the film formed was annealed in hydrogen atmosphereat 350° C. for 35 minutes.

FIG. 3 shows the results of the dielectric breakdown test of the siliconoxide films of 1000 Å thick that had been formed on high-resistancesilicon wafers using the present apparatus. Over the silicon oxide film,formed was a 1 mmφ-aluminum electrode and the relation between thevoltage and the current was plotted. FIG. 3(C) indicates the film thathad been formed on the substrate without any particular treatment of thesubstrate prior to the filming, from which it is noted that thebreakdown voltage of the film is low. The films of FIG. 3(A) were formedas follows: After the substrates were set in the chamber, they wereheated at 300° C. and exposed to the plasma atmosphere generated byintroducing 400 SCCM of oxygen and from 0 to 5 SCCM of TCE. The totalpressure of the atmosphere was 5 Pa, and the RF power was 150 W. Theplasma exposure was carried out for 10 minutes. (During the step, nofilm was formed by the gaseous reaction.) After the plasma exposure, thesilicon oxide films of FIG. 3(A) were formed, and they showed a highbreakdown voltage.

The films of FIG. 3(B) were formed as follows in the same manner as inFIG. 3(A) except that the flow rate of TCE in the filming step waschanged to 4 SCCM or more, for example 5 SCCM. As shown, they had a lowbreakdown voltage. From these results, it has been found that the TCEconcentration for the filming has the optimum value.

FIG. 4(A) shows the result of the bias/temperature test, as one exampleof the reliability tests, of the insulating films formed in thisexample, indicating the relation between the fluctuations (V_(FB)) ofthe flat band voltage (V_(FB)) and the pre-treatment, if any, of thesubstrates. In the bias/temperature test, a voltage of +17 V wasimparted to the sample at 150° C. for one hour and the C-Vcharacteristic of the sample was measured at room temperature. Next, avoltage of -17 V was imparted to the same sample at 150° C. for one hourand the C-V characteristic thereof was also measured at roomtemperature. The difference in V_(FB) between the two measurements wasobtained to be V_(FB).

In FIG. 4(A), the substrate of the sample (a) was not pre-treated.V_(FB) of the sample (a) was about 5 V and was relatively large.However, the problem was solved by pre-treating the substrate. Thesubstrates of the samples (b) and (c) were pre-treated under theconditions mentioned below.

    ______________________________________                                        Sample             (b)        (c)                                             ______________________________________                                        Temperature of Substrate                                                                         300° C.                                                                           300° C.                                    TCE/Oxygen                      0/400    0.5/400                              RF Power                        150 W    150 W                                Time for Treatment              10 min 10 min                               ______________________________________                                    

From FIG. 4(A), it is understood that the reliability of the insulatingfilm was improved much more by pre-treating the substrate using TCE.

The same improvement may also be attained by annealing the insulatingfilm formed. The annealing of the film was carried out in argon of oneatmospheric pressure at 300 to 570° C. for one hour. The relationbetween the annealing temperature and V_(FB) is shown in FIG. 4(B), fromwhich it is noted that V_(FB) was significantly reduced when the filmwas annealed at temperatures not higher than 450° C., while it becamegradually constant when the annealing temperature was being near to 600°C. From the result, it was clarified that the annealing of theinsulating film formed is effective in improving the reliability of thefilm.

On the basis of the results obtained from the above-mentionedexperiments, a TFT sample was produced. The flow sheet for producing itis shown in FIG. 2. First, the silicon oxide film 202 of 2000 Å thickwas formed, as a subbing film, on the substrate (Corning 7059) 201, bypositive column plasma CVD using TEOS, oxygen and TCE as raw materials.The apparatus used herein was same as that shown in FIG. 1. The mainconditions for the filming were as follows:

    ______________________________________                                        Temperature of Substrate:                                                                             300° C.                                          Whole Pressure:               5 Pa                                            Mixed Gas:                                                                  TOES:         12 SCCM                                                           Oxygen:   300 SCCM                                                            TCE:      2 SCCM                                                            RF Power:               75 W                                                  ______________________________________                                    

Next, an amorphous silicon film of 500 nm thick was deposited thereoverby plasma CVD, and this was patterned to form the island silicon region203. This was allowed to stand in nitrogen atmosphere at 400° C. for 30minutes to remove hydrogen therefrom. Next, this was annealed with alaser ray, as shown in FIG. 2(A), to crystallize the silicon region. Asthe laser, used was a KrF excimer laser (having a wavelength of 248 nmand a pulse width of 20 nsec). The energy density was from 200 to 350mM/cm². During the irradiation of the laser rays, the substrate was keptat 300 to 500° C., for example 450° C.

Afterwards, the silicon oxide film 204 of 1000 Å thick was formed tocover the island silicon region 203, as a gate-insulating film, bypositive column plasma CVD using TEOS, oxygen and TCE as raw materials,as shown in FIG. 2(B). Prior to the filming, the substrate waspre-treated, using the same apparatus as in Example 1. The mainconditions for the pre-treatment were as follows:

    ______________________________________                                        Temperature of Substrate:                                                                            300° C.                                           Whole Pressure:                 5 Pa                                          Mixed Gas:                                                                  Oxygen:     400 SCCM                                                            TCE:      0.5 SCCM                                                            RF Power: 150 W                                                             Time for Treatment:    10 minutes                                             ______________________________________                                    

After the pre-treatment, the film 204 was formed. The main condition forthe filming were mentioned below. After the filming, the film formed wasannealed in argon atmosphere at 550° C. for one hour.

    ______________________________________                                        Temperature of Substrate:                                                                             300° C.                                          Whole Pressure:               5 Pa                                            Mixed Gas:                                                                  TEOS:        15 SCCM                                                            Oxygen:   300 SCCM                                                            TCE: 2 SCCM                                                                 RF Power:               75 W                                                  ______________________________________                                    

Next, a 2% silicon-doped aluminum film of 6000 Å thick was depositedover the film and this was patterned to form the gate electrode 205.Then, impurity ions (phosphorus or boron) were introduced into theregion 203 in a self-ordered manner by plasma doping, using the gateelectrode 205 as the mask, to form the impurity regions 206 and 207, asshown in FIG. 2(C). The area into which the impurities had not beenintroduced became the channel-forming region 208. Since the doping wasconducted through the gate-insulating film, it needed an acceleratedvoltage of 80 kV for phosphorus and 65 kV for boron. The dose amount wassuitable from 1×10¹⁵ to 4×10¹⁵ cm⁻².

Next, the impurities were activated also by annealing with laser rays,as shown in FIG. 2(D). As the laser, used was the KrF excimer laser(having a wavelength of 248 nm and a pulse width of 20 nsec). The energydensity was from 200 to 350 mJ/cm². During the irradiation of the laserrays, the substrate may be kept at 300 to 500° C. After the irradiationof the laser rays, this was annealed at 350° C. in hydrogen atmospherehaving a partial pressure of from 0.1 to 1 atmospheric pressure for 35minutes.

Next, the silicon oxide film 209 of 5000 Å thick was deposited thereoveras an interlayer insulating film. The silicon oxide film 209 was formedby positive column CVD, using TEOS, oxygen and TCE as raw materials. Theapparatus used for the filming was the same as in Example 1. The mainconditions for the filming were as follows:

    ______________________________________                                        Temperature of the Substrate:                                                                         300° C.                                          Whole Pressure:                     5 Pa                                      Mixed Gas:                                                                  TEOS:        30 SCCM                                                            Oxygen:   300 SCCM                                                          RF Power:               100 W                                                 ______________________________________                                    

Afterwards, the contact holes 210 and 122 were formed through theinterlayer insulating film, and the electrodes 212 and 213 were formedas a source and a drain, respectively, of TFT, using aluminum. In placeof aluminum, also usable are titanium and titanium nitride. In thismanner mentioned above, TFT was completed. The production yield of TFTwas extremely improved, since the step coverage of the gate-insulatingfilm was improved and the reliability of the gate-insulating film wasimproved.

As mentioned above in detail, the silicon oxide film of the presentinvention has sufficient reliability as a gate-insulating film. Inaddition, it has become obvious that the present invention contributesto not only the improvement in the reliability of the film but also theimprovement in the production yield. Moreover, the mass-producibility ofthe device of the present invention may be improved, using the positivecolumn plasma CVD apparatus such as that employed in the example. Thus,the present invention is useful as an industrial invention.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

We claim:
 1. An insulating film comprising silicon oxide formed over aglass substrate,wherein said insulating film includes halogen at aconcentration of 1×10¹⁷ to 5×10²⁰ cm⁻³ and carbon at a concentration of5×10¹⁹ cm⁻³ or less which are detected by secondary ion massspectroscopy.
 2. The film according to claim 1, wherein the halogen isfluorine or chlorine.
 3. The film according to claim 1, wherein saidinsulating film includes carbon at a concentration of 1×10¹⁸ cm⁻³ orless which is detected by the secondary ion mass spectroscopy.
 4. Thefilm according to claim 1, wherein said insulating film is a gateinsulating film.
 5. The film according to claim 1 wherein saidinsulating film is an insulating film in a thin film transistor.
 6. Thefilm according to claim 1, wherein said insulating film covers an evensurface over the glass substrate.
 7. A semiconductor device comprising:acrystalline semiconductor island formed over a glass substrate; and aninsulating film including silicon oxide formed to cover the crystallinesemiconductor island, wherein said insulating film includes halogen at aconcentration of 1×10¹⁷ to 5×10²⁰ cm⁻³ and carbon at a concentration of5×10¹⁹ cm⁻³ or less.
 8. The device according to claim 7, wherein theconcentrations of halogen and carbon are detected by secondary ion massspectroscopy.
 9. The device according to claim 7, wherein said halogenis fluorine or chlorine.
 10. The device according to claim 7, whereinsaid insulating film is formed by plasma chemical vapor deposition usingan organic silane.
 11. A thin film transistor comprising:a crystallinesemiconductor island formed over a glass substrate; a silicon oxide filmformed to cover the crystalline semiconductor island; and a conductivefilm including at least one of aluminum, titanium, and titanium nitride,said conductive film being formed on the silicon oxide film, wherein thesilicon oxide film includes halogen at a concentration of 1×10¹⁷ to5×10²⁰ cm⁻³ and carbon at a concentration of 5×10¹⁹ cm⁻³ or less. 12.The transistor according to claim 11, wherein said halogen is fluorineor chlorine.
 13. The transistor according to claim 11, wherein saidsilicon oxide film is formed by plasma chemical vapor deposition usingan organic silane.
 14. A thin film transistor comprising:a crystallinesemiconductor island formed over a glass substrate; a gate insulatingfilm including silicon oxide formed on the crystalline semiconductorisland; and a gate electrode formed on the insulating film, wherein saidgate insulating film includes halogen at a concentration of 1×10¹⁷ to5×10²⁰ cm⁻³ and carbon at a concentration of 5×10¹⁹ cm⁻³ or less. 15.The transistor according to claim 14, wherein said halogen is fluorineor chlorine.
 16. The transistor according to claim 14, wherein said gateinsulating film is formed by plasma chemical vapor deposition using anorganic silane.